At least two types of cellular radiotelephone systems are operating in the United States, an analog type and a digital type. The analog type, also know in the art as the advanced mobile phone system (AMPS), was the sole cellular system until capacity demands required that another system having greater capacity be utilized. The digital type system, presently implemented using either code division multiple access (CDMA) or time division multiple access (TDMA) schemes, was invented to relieve the capacity problems of AMPS.
Mobile radiotelephones that are designed to operate in one of the digital systems must also have the capability of operating in AMPS. This allows a mobile radiotelephone user to roam from one geographical area with AMPS to another geographical area with one of the digital schemes. Or the user can decide to switch from one type of system to the other while remaining in the same geographical area due to lack of capacity in the other system.
These cellular systems have greatly increased the mobility of the public. Computers and facsimile machines can now be coupled to mobile telephones, thus allowing data communications, in addition to voice communications, from any location having a cellular system installed.
One problem with coupling a computer with a radiotelephone is that, in an analog system, the data from the computer must be converted into an analog format to be compatible with the AMPS format. This requires a modem between the radiotelephone and the computer, adding weight and expense to the configuration. Yet when the radiotelephone switches to a digital system, the modem is not required since the digital data from the computer can just be reformatted into the digital standard's format before transmission. In this case, the user must remember to disconnect the computer from the modem and reconnect the computer to the radiotelephone. There is a resulting need for a data communications apparatus that does not require the user to reconfigure the connections when changing cellular systems.